Childhood obesity is one of the most prominent public health concerns in the U.S. Experts believe that environmental and policy changes will likely be essential components of efforts to address this epidemic. But the literature on the environment is largely cross-sectional and lacks random assignment, so experts have called for quasi-experimental methods, particularly natural experiments, to produce the causal estimates necessary to inform policy. Our proposed study leverages one such natural experiment where military children relocate to new schools and neighborhoods as a result of their military parents' transfers. Such relocations, or Permanent Change of Station (PCS), typically occur every 2-3 years and are determined by the military to support the unique missions and units. For our purposes, PCS moves imply that the locations of military personnel at any given point in time and the changes in their locations over time are largely unrelated to family members' diet and activity behaviors. Consequently, their children are arguably exogenously assigned to school and neighborhood food and physical activity (hereafter, FPA) environments. We propose to collect and analyze longitudinal data on a sample of middle school children from military families to examine the effects of arguably exogenous changes in FPA environments resulting from PCS moves on their obesity and related behaviors. Our specific aims are as follows: 1.Describe the FPA environments in the schools and neighborhoods where our sample military children study and live. 2.Examine cross-section ally whether school and neighborhood FPA environments of military children are associated with diet, physical activity, sedentary behavior, and body composition. 3.Analyze how changes in FPA environments resulting from PCS moves influence changes in diet, physical activity, sedentary behavior, and body composition for the same child longitudinally. Our study will make a significant contribution to the literature on the effects of FPA environment on children's health behaviors and outcomes. The quasi-experimental design and longitudinal analyses address common concerns about self-selection that have limited causal inference. We will also collect baseline data on a sample of civilian children to compare to our military sample and produce generalizable estimates using propensity score weighting. Our proposed study is also innovative in other ways. For example, we provide a more complete picture of how the environment influences energy balance and body composition instead of focusing on just diet or physical activity. Moreover, we focus on adolescence, when the impact of the built environment may be a strong determinant of physical activity and diet. Finally, the focus on children of Army enlistees is useful because these families are more likely to be from racial/ethnic minority groups, for whom the risk of obesity is greater, but not well-represented in previous studies.